The present invention relates to cardiac pacers. Many different types of cardiac pacers are known, such as fixed rate pacers and demand or ventricular inhibited pacers. In fixed rate pacers, circuitry generates a train of stimulating pulses of constant period or interval (approximating the interval between natural heartbeats), and these pulses are coupled to the heart to force its rhythm. In a demand type of pacer, the natural activity of the heart is sensed (usually at the ventricle), and this information is used to inhibit the pacer circuitry from generating a pulse. In this type of pacer, it may be said that the electronic circuitry does not "compete" with a natural heartbeat. The interval between the last natural heartbeat to be sensed and the first stimulating pulse is referred to as the "escape interval". It is common practice to have the escape interval in this type of pacer adjusted to a value which will maintain a desired rate of pacing in the absence of naturally-occurring heartbeats.
There are other types of pacers, and there are many variations within each broad classification, such as so-called "hysteresis" pacers which have different escape intervals following sensed or paced beats. For example, the first escape interval may be relatively long, and subsequent escape intervals may be shorter.
Another type of pacer, namely that disclosed in U.S. Pat. No. 3,662,759, granted May 16, 1972, senses a natural heartbeat, and if it occurs, generates a non-stimulating "tracking" pulse in synchronism with a naturally occurring heartbeat. If no natural heartbeat is sensed, then at the end of the escape interval a pulse of greater energy, sufficient to stimulate the heart, is generated. In a non-competing pacer of this type, the tracking pulse may communicate to a physician (over a telephone communication line), that the heart is working in natural sinus rhythm, and that the implanted circuitry is also operational and functioning properly. If, during an examination, the heart is functioning normally and the pacer circuitry quiescent, the tracking pulse assures an examining physician that the circuitry is operative, the power supply functioning, and the electrode system intact (the tracking pulse usually being sensed right at the heart).
Still another type of pacer system is referred to as a threshold tracking pacer. In these systems, the pacing circuitry tries to measure the threshold which is just sufficient to stimulate the heart. It will be appreciated that the stimulating threshold not only varies from person to person, but may vary with time for the same person, and it may change for different electrode systems. Once a stimulating threshold is measured, a system of this type may increase the stimulation by a fixed amount for subsequent pulses, or it may be programmed to seek the threshold anew at fixed times, or each time that natural sinus rhythm is lost. In pacers of this type, it is important to determine when a stimulating pulse is or is not actually stimulating the heart. In this context, "stimulation" means that the pulse transmitted to the heart electrode causes depolarization and contraction of one of the heart chambers, for example, the ventricle. The determination that a stimulating pulse has produced cardiac depolarization or "capture" is referred to as "capture verification". A threshold tracking pacer system is disclosed in an article "Automatic Threshold Tracking Pacemaker", Preston and Bowers, MEDICAL INSTRUMENTATION, Vol. 8, No. 6, November-December, 1974. The article does not disclose the circuitry used, but it describes four separate electrode systems. The first system used four electrodes--two for pacing and two different electrodes for sensing capture. A second electrode system used three electrodes, two of which were positioned in the heart by a catheter, and a third large electrode was remote (perhaps in the abdominal region where artificial pacers are normally implanted). Pacing was from the distal tip electrode to the remote electrode, and sensing was from the second embedded electrode to the remote electrode. The third electrode used a bipolar electrode catheter and a large reference electrode attached to the body surface and, as described, exhibited sensing problems at high current levels. The fourth electrode system used a bipolar catheter with both electrodes used for pacing and sensing. A bipolar catheter is one which has two electrode surfaces, electrically isolated, at the distal end of a catheter. This electrode system, as indicated, produced sensing difficulties due to residual after-potential and electrode polarization. The authors conclude that a three-electrode system is necessary for proper sensing.
Electrode polarization is a well-known phenomenon caused by the interface between the metal surface of the electrode and the ions in solution in the body tissue with which the electrode communicates. In this connection "polarization" refers to a residual voltage resulting from the accumulation of charge at the interface between the electrode metal surface and the electrolyte or body fluid; and it is not to be confused with depolarization of the cells of the heart which accompanies contraction. The polarization voltage may be small, of the order of 0.5-1.7 volts, but it is a significant factor in capture verification because the sensing system is trying to detect the presence of an R wave voltage of the order of 10 millivolts to verify capture, and this signal is small in comparison with normal polarization voltages. In addition to the difference in magnitude between a typical polarization voltage and a stimulated R wave, a polarization voltage does decay with time, and the slope of the decaying voltage even further complicates the detection of a stimulated R wave, as will be further discussed within. Thus, the effect of electrode polarization, in terms of an electrical equivalent circuit, is that it appears to the driving circuitry that the electrode has capacitance which stores some residual charge after stimulation, and reference will be made, then, to an electrode capacitance. The value of the capacitance depends on the material of the electrode and its size.
Another cardiac pacer which senses capture and decreases the amplitude of a succeeding stimulating pulse if a stimulated heartbeat is sensed is disclosed in U.S. Pat. No. 3,757,792, issued Sept. 11, 1973. In this patent, if the circuitry does not sense a stimulated R wave after a stimulating pulse is transmitted to the heart, the next succeeding stimulating pulse is increased in amplitude by a predetermined amount. As stated in the patent, separate pairs of electrodes are used for stimulating and for sensing to avoid the problem of electrode polarization and the resulting refractory period.
The desirability of a single electrode system for both stimulating and sensing capture verification is apparent because of the resulting simplicity in electrode design and operation. However, to our knowledge, workers in the art have been unable to produce an operable, reliable system for indicating capture verification using only a single electrode implanted in the heart for both sensing and detection. The present invention is directed to such a system. The second electrode is remotely located--for example, in the abdomen where the circuitry is normally implanted.